1. Field of the Invention
The present invention relates to an MEMS (micro electromechanical system)-scanning mirror device and a method for manufacturing the mirror device.
2. Description of the Related Art
Along with the recent improvement in the multifunctionality and operation speed of optical devices, high-speed optical path switching and vector drawing of desired patterns have been required. For example, in an electronic distance meter, in order to compensate for measurement errors, alternate switching is performed between an external optical path for measuring a distance from the electronic distance meter to an external target and an internal optical path provided inside the device.
For example, a variable optical attenuator in a distance measuring equipment disclosed in Japanese Patent Application Publication No. 2008-65293 requires optical path switching for attenuation. Moreover, in capturing a target by the electronic distance meter, a light beam has to be projected at a specified angle. Furthermore, in order to draw a desired pattern, a line drawing display device using laser beam scanning is required to be able to perform optical scanning corresponding to the pattern.
In either case, it is important to improve the operation speed and power consumption. For that purpose, application of an MEMS-scanning mirror to changing a light reflection direction and scanning has been examined. Moreover, in addition to the improvement in the operation speed of an actuator and the power consumption thereof, improvement in the specularity of a mirror portion configured to reflect light and in manufacturing yield has been the primary focus in this development field. Here, the specularity corresponds to the roughness of a surface of the mirror portion, and a smooth surface having small surface roughness is said to have high specularity.
FIG. 6 is a schematic view showing a conventional example of an MEMS-scanning mirror 27 having stepped electrostatic comb actuators. As shown in FIG. 6, certain steps are provided between movable combteeth 27e and fixed combteeth 27fA and 27fB.
The MEMS-scanning mirror 27 having the stepped electrostatic comb actuators includes a disc-shaped mirror plate 27a. In the mirror plate 27a, a pair of radially-extended shaft portions 27b are formed. The pair of shaft portions 27b are connected to fixed portions 27d through spring portions 27c, respectively.
The movable combteeth 27e are formed in a middle of each of the shaft portions 27b. The fixed combteeth 27fA and 27fB are allowed to face the movable combteeth 27e. The movable combteeth 27e and the fixed combteeth 27fA and 27fB form a pair of left and right electrostatic actuators.
The pairs of left and right electrostatic actuators are used to drive and rotate the mirror plate 27a. Specifically, the mirror plate 27a is driven to be rotated in an arrow F direction by applying a voltage to the pairs of left and right fixed combteeth 27fA and 27fB. Note that there are two kinds of drive systems: DC drive for performing rotation from a first angle to a second angle by applying a DC voltage between the movable combteeth and one of or both of the left and right fixed combteeth and resonant drive for performing rotation based on resonance by applying a periodic voltage such for example as an AC voltage.
There have been known various methods for manufacturing of the MEMS-scanning mirror having the stepped electrostatic comb actuators. For example, U.S. Pat. No. 6,758,983 discloses a process for manufacturing an MEMS-scanning mirror which includes two layers of silicon wafers and can be driven to be rotated around one axis. The process includes the steps of    (a) forming fixed combteeth structures in a surface of a first silicon wafer;    (b) bonding a second silicon wafer to the surface of the first silicon wafer where the fixed combteeth structures are formed;    (c) after the step (b), reducing a thickness of the second silicon wafer to a predetermined thickness and then performing grinding or polishing to obtain specularity;    (d) after the step (c), removing a portion corresponding to a lower portion of a mirror plate from a surface of the first silicon wafer, the surface being opposite to the surface where the fixed combteeth structures are formed;    (e) after the step (d), forming movable combteeth and the mirror plate in the second silicon wafer; and    (f) after the step (e), removing an oxide layer between the first and second silicon wafers to obtain an MEMS-scanning mirror having stepped electrostatic comb actuators. Note that the mirror plate may be coated with gold, aluminum or the like by deposition or the like in order to improve its optical characteristics.
In addition, U.S. Pat. No. 6,914,710 discloses a process for manufacturing an MEMS-scanning mirror which can be driven to be rotated around two axes. The process includes the steps of:    (a) forming a lower layer structure of the MEMS-scanning mirror having fixed combteeth, in a device layer in a silicon-on-insulator (SOI) wafer;    (b) forming a concave portion in another silicon wafer;    (c) after the step (b), bonding the device layer of the SOI wafer to the silicon wafer in a state where the lower layer structure and the concave portion face each other;    (d) after the step (c), reducing a thickness of a handle layer of the SOI wafer to a predetermined thickness and then performing mirror finishing;    (e) after the step (d), forming an upper layer structure of the MEMS-scanning mirror having movable combteeth and a mirror plate, in the handle layer; and    (f) after the step (e), removing an exposed silicon oxide layer to obtain stepped electrostatic comb actuators.
Note that the mirror is completed by forming a film of gold with chromium as an underlayer, on the mirror plate formed in the step (e).
Moreover, there has also been known a technology related to an MEMS structure described in U.S. Pat. No. 6,641,273, for example.
However, in each of the conventional MEMS-scanning mirrors having the stepped electrostatic comb actuators described in U.S. Pat. Nos. 6,758,983 and 6,914,710, for example, the mirror plate is formed on a top surface of the device. Thus, the mirror has the following problems, which lead to a situation where it is difficult to obtain a mirror having high specularity and to improve manufacturing yield.
Specifically, in formation of the mirror plate, processing such as grinding, lapping and polishing has heretofore been performed on the uppermost layer of the device in order to improve the specularity. In this event, as in the case of the manufacturing method disclosed in U.S. Pat. No. 6,914,710, for example, when structures such as combteeth and hollows are previously formed in a lower portion of the silicon wafer to be subjected to such processing, especially when a hollow is formed below the mirror plate, it is difficult to secure uniformity of the processing. As a result, scratches are left on the mirror surface, which makes it difficult to obtain high specularity with good yield.
Furthermore, in conventional cases, since the mirror portion is formed on the top surface of the device, damage to the mirror portion due to handling, internal stress and the like, scratches on the mirror surface, and the like are likely to occur in a processing step subsequent to processing steps of mask formation, etching, mask removal, film formation and the like after formation of the mirror portion. These arc also obstacles to achievement of good specularity and high manufacturing yield.
Such problems are significant in a subsequent processing step particularly when a hollow is formed below the mirror plate portion.
Moreover, a concrete manufacturing method is not disclosed in U.S. Pat. No. 6,641,273, for example. Therefore, it is impossible to achieve a high manufacturing yield according to a manufacturing method including: exposing the surface of the mirror by removing silicon oxide films covering the entire front and back surfaces thereof; forming stepped combteeth structures with a handle layer present below the mirror plate; or exposing the surface of the mirror plate by removing silicon oxide films after forming the stepped combteeth structures with the handle layer is present below the mirror plate.